The disclosed embodiments relate to a method and to apparatus for treating a synthesis gas or “syngas”. It also relates to a system for treating waste or biomass, which system is equipped with such treatment apparatus.
Methods are known that make it possible to obtain a syngas
Autothermal gasification is, for example, a well-known method whose main mechanism seeks, in oxygen under-stoichiometry and by injecting steam, to decompose carbon chains such as those contained in biomass, forest residues, household and hospital waste, soiled wood, and any other waste having organic potential, with a view to obtain a syngas that is combustible and suitable for use in recycling.
The definite advantage procured by gasification is that, in the absence of full combustion, the organic fraction decomposes in the form of a combustible gas (fuel gas) whose “lower combustion value” (LHV) or “net calorific value” (NCV) increases with decreasing presence of carbon dioxide, of water vapor and of nitrogen, these molecules being inefficient when used in recycling as means to generate electricity, as biofuel, or in organic chemistry.
In addition, the presence of tars and of particles containing solid carbon (Char) in the syngas constitutes a major drawback for the elements downstream from the gasification reactor. Such particles (tars and char) can condense easily in the syngas treatment pipes, thereby giving rise to obstruction of said pipes and to risks of fire starting spontaneously on opening the pipes for maintenance purpose. Furthermore, such solid elements can build up on gas turbine blades and in gas engines, thereby lastingly reducing performance thereof and increasing frequency of servicing and maintenance thereof.
The vast majority of technologies for preparing syngas prior to use in recycling consist in removing the solid particles (tars and char) as soon as the syngas leaves the autothermal gasification reactor, such removal being by filtration (cyclonic filter, bag filter, electrostatic filter), condensation (water scrubber, oil scrubber), or cracking the solid particles by using catalytic reactions (using pure oxygen and steam) or indeed by high-temperature reforming (using pure oxygen).
The constraints imposed by the design of autothermal gasification reactors, the refractory materials of which they are made, and the presence of moving elements (rakes made of refractory steel, sands, metal balls) making it possible to homogenize the load, mean that it is difficult for temperature higher than 1000° C. to be withstood.
Treatment capacity is also limited by the variability of the incoming matter in terms of composition and grain-size, and by its humidity level and its mineral content, and in particular its heavy metal content.
Those factors result in gasification methods being performed at temperatures lying in the range 600° C. to 1000° C. in order to be economically viable.
Therefore, it is necessary to accept obtaining a syngas with a mediocre NCV because although it admittedly contains carbon monoxide and Hydrogen as dominant species, it also contains by-products that cannot be used in recycling, such as carbon dioxide, water vapor, and nitrogen.
As regards the solid particles (char) in suspension in the syngas, the approach consists in extracting them from the syngas and in recycling them back into the reactor as a thermal energy source. That action, which consists in removing that carbon potential initially available in the organic material to be treated from the gasification method, gives rise to a limitation of the carbon efficiency, the direct consequence of which is a limitation in the NCV.
The risks are also environmental and health ones for the operators. Extracting solid particles in suspension in the syngas generate residual sludge in the syngas treatment system. That sludge then needs to be removed from the site to landfills or industrial waste incinerators. The operators are thus exposed to carcinogenic products during maintenance of the scrubbers or of the sewage treatment plants.
The flows of multi-phase waste, such as the mixture with the ashes at the bottom of the gasification reactor, and the residual sludge (tars/char/water/oil) coming from the treatment line for treating the syngas prior to use in recycling represent a considerable economic cost as regards removing them from the site to landfill or destruction sites.
Another gasification method is known, namely direct gasification using plasma. That method consists in attacking the organic material directly with plasma so as to convert it into a high-purity, high-temperature syngas.
The general configuration of such a method is usually as follows: one or more plasma deliver one or more plasma flows into a furnace fed with materials to be gasified and/or to be vitrified. The furnace then hosts thermochemical reactions for transforming the materials fed in, under the direct action and/or the indirect action of the plasma flow. The liquid and gas phases that result from the synthesis or from the plasma treatment are then recovered for any subsequent treatment implementing exiting techniques.
The essential components of such installations, except for the plasma tools implemented in the method, comprise apparatus for injecting solid matter in powder-form, or for injecting liquid, or indeed for injecting semi-liquid substances (sewage plant sludge, petroleum sludge).
Directly attacking incoming matter having a high organic content is not economically viable insofar as extracting humidity fraction contained in the organic material gives rise to electricity consumption which is less pertinent than using thermal energy recycled from the method.
Furthermore, the use of a single furnace lined with refractory material and that must cope with the liquid, solid, and gas phases gives rise to operating modes that limits the flow rate of incoming matter or the variability of the incoming matter.
It is the portion of the furnace that has to cope with the liquid mineral that withstands refractory lining corrosion/impregnation for the shortest amount of time. That portion is thus the floor of the furnace, which requires the gasification method to be stopped in order to perform maintenance on said floor.
The mixture of the plasma and of the materials to be gasified and/or to be vitrified does not include all of the materials, the thermochemical treatment mainly concerning an indirect process (thermal radiation coming from the refractory walls of the furnace that are heated to high temperatures under the action of plasma). Therefore the energy transfer between the plasma and the materials is not optimized.
Moreover, manufacturing the furnace requires the use of refractory materials whose erosion is very sensitive to the variations in temperature generated by varying energy needs corresponding to the variable chemical composition of the incoming materials, and by the periodic removal of the plasma tool for the purpose of changing electrodes. In addition, the chemical natures of the gases resulting from the plasma treatment can also limit the life of the refractory linings, in particular when said gases contain chlorine.